Chapter IVDose estimates*

The exposure of the population as a result of the accident resulted
in two main pathways of exposure. The first is the radiation dose to
the thyroid as a result of the concentration of radioiodine and similar
radionuclides in the gland. The second is the whole-body dose caused
largely by external irradiation mainly from radiocesium.

The absorbed dose to the whole body is thought to be about 20 times
more deleterious, in terms of late health effects incidence, than the
same dose to the thyroid (IC90).

The population exposed to radiation following the Chernobyl accident
can be divided into four categories: (1) the staff of the nuclear power
plant and workers who participated in clean-up operations (referred
to as "liquidators"); (2) the nearby residents who were evacuated
from the 30-km zone during the first two weeks after the accident; (3)
the population of the former Soviet Union, including especially the
residents of contaminated areas; and (4) the population in countries
outside the former Soviet Union.

A number of liquidators, estimated at up to 600 000 (civilian and
military according to laws promulgated in Belarus, the Russian Federation
and Ukraine) took part in mitigation activities at the reactor and within
the 30-km zone surrounding the reactor. The most exposed workers were
the firemen and the power plant personnel during the first days of the
accident. Most of the dose received by the workers resulted from external
irradiation from the fuel fragments and radioactive particles deposited
on various surfaces. Of particular interest are the 226 000 recovery
operation workers who were employed in the 30-km zone in 1986-1987,
as it is in this period that the highest doses were received. The remainder
of the recovery operation workers, who generally received lower doses,
amounted to about 400 000 (UN00).

About 116 000 people were evacuated during the first days following
the accident, mainly from the 30-km zone surrounding the reactor. Prior
to evacuation, those individuals were exposed to external irradiation
from radioactive materials transported by the cloud and deposited on
the ground, as well as to internal irradiation essentially due to the
inhalation of radioactive materials in the cloud.

The relative contributions to the external whole-body dose from the
main radionuclides of concern for that pathway of exposure and during
the first few months after the accident are shown in Figure 8. It is
clear that 132Te played a major role in the first week after the accident,
and that, after one month, the radiocaesiums (134Cs and 137Cs) became
predominant. Subsequently, however, 134Cs decayed to levels much lower
than those of 137Cs, which became after a few years the only radionuclide
of importance for practical purposes. It is usual to refer to 137Cs
only, even when the mix of 134Cs and 137Cs is meant, because the values
for the constituents can be easily derived from those for 137Cs.

With regard to internal doses from inhalation and ingestion of radionuclides,
the situation is similar: radioiodine (131I) was important during the
first few weeks after the accident and gave rise to thyroid doses via
inhalation of contaminated air, and, more importantly, via consumption
of contaminated foodstuffs, mainly cow's milk. After about one month,
the radiocaesiums (134Cs and 137Cs) again became predominant, and, after
a few years, 137Cs became the only radionuclide of importance for practical
purposes, even though 90Sr may in the future play a significant role
at short distances from the reactor.

Among the population of the former Soviet Union, it is usual to single
out the residents of the contaminated areas, defined as those with 137Cs
deposition levels greater than 37 kBq/m2. About 5 million people live
in such areas. Of special interest are the inhabitants of the spots
with 137Cs deposition levels greater than 555 kBq/m2. In those areas,
called "strict control zones", protection measures are applied,
especially as far as control of consumption of contaminated food is
concerned. In 1998, 42 554 measurements were performed in the Federal
Republic of Russia, and the national authorities are planning to maintain
such controls beyond 2000 (Bo99). In 1986, shortly after the accident,
the All-Union Dose Registry (AUDR) was set up by the Soviet Government
to record medical and dosimetric data on the population groups expected
to be the most exposed: (1) the liquidators, (2) the evacuees from the
30-km zone, (3) the inhabitants of the contaminated areas, and (4) the
children of those people. In 1991, the AUDR contained data on 659 292
persons. Starting from 1992, national registries in Belarus, Russian
Federation, and Ukraine replaced the AUDR.

Outside the former Soviet Union, the radionuclides of importance are,
again, the radioiodines and radiocaesiums, which, once deposited on
the ground, give rise to doses from ingestion through the consumption
of foodstuffs. Deposited radiocaesium is also a source of long-term
exposure from external irradiation from the contaminated ground and
other surfaces. Most of the population of the Northern hemisphere was
exposed, in varying degrees, to radiation from the Chernobyl accident.
The 137Cs deposition outside the former Soviet Union ranged from negligible
levels to about 50 kBq/m2.

The liquidators

Most of the liquidators can be divided into two groups: (1) the people
who were working at the Chernobyl power station at the time of the accident
viz. the staff of the station and the firemen and people who went to
the aid of the victims. They number a few hundred persons; and (2) the
workers, estimated to amount up to 600 000, who were active in 1986-1990
at the power station or in the zone surrounding it for the decontamination,
sarcophagus construction and other recovery operations.

On the night of 26 April 1986, about 400 workers were on the site
of the Chernobyl power plant. As a consequence of the accident, they
were subjected to the combined effect of radiation from several sources:
(1) external gamma/beta radiation from the radioactive cloud, the fragments
of the damaged reactor core scattered over the site and the radioactive
particles deposited on the skin, and (2) inhalation of radioactive particles
(UN88).

All of the dosimeters worn by the workers were over-exposed and did
not allow an estimate of the doses received. However, information is
available on the doses received by the 237 persons who were placed in
hospitals and diagnosed as suffering from acute radiation syndrome.
Using biological dosimetry, it was estimated that 41 of these patients
received whole-body doses from external irradiation in the range 1-2
Sv, that 50 received doses between 2 and 4 Sv, that 22 received between
4 and 6 Sv, and that the remaining 21 received doses between 6 and 16
Sv. In addition, it was estimated from thyroid measurements that the
thyroid dose from inhalation would range up to about 20 Gy, with 173
individuals in the 0-1.2 Gy range and seven workers with thyroid doses
greater than 11 Gy (UN88). Internal exposure of those workers was mainly
due to 131I and shorter-lived radioiodines, the median value of the
ratio of the internal thyroid dose to the external effective dose was
estimated to be 0.3 Gy per Sv. The doses resulting from intakes of other
radionuclides was estimated to about 30 mSv for the early months following
the accident and 85 mSv for committed dose (UN00).

The second category of liquidators consists of the large number of
adults who were recruited to assist in the clean-up operations. They
worked at the site, in towns, forests and agricultural areas to make
them fit to work and live in. About 600 000 of individuals participated
in this work. Initially, about 240 000 of those workers came from the
Soviet armed forces, the other half including personnel of civil organisations,
the security service, the Ministry of Internal Affairs, and other organisations.
The total number of liquidators has yet to be determined accurately,
since only some of the data from some of those organisations have been
collected so far in the national registries of Belarus, Russia, Ukraine
and other republics of the former Soviet Union (So95). Also, it has
been suggested that, because of the social and economic advantages associated
with being designated a liquidator, many persons have contrived latterly
to have their names added to the list. To day the total number of recovery
operation workers recorded in the registries appears to be about 400
000 well below the figure of 600 000, which corresponds to the number
of people who have received special certificates confirming their status
as liquidators. The workers were all adults, mostly males aged 20-45
years.

There are only fragmented data on the doses received by these liquidators.
Attempts to establish a dosimetric service were inadequate until the
middle of June of 1986, until then, doses were estimated from area radiation
measurements. The doses to the recovery operation workers who participated
in mitigation activities within two months after the accident are not
known with much certainty. The liquidators were initially subjected
to a radiation dose limit for one year of 250 mSv. In 1987 this limit
was reduced to 100 mSv and in 1988 to 50 mSv (Ba93). The registry data
show that the average recorded doses in the three national registries
decreased from year to year, being about 170 mSv in 1986, 130 mSv in
1987, 30 mSv in 1988 and 15 mSv in 1989 (Se95a). It is, however, difficult
to assess the validity of the results as they have been reported since
these statistics indicates that the dose is known for only 52% of workers
for the period 1986-1989, with a lower percentage, 45% for the first
year. Moreover uncertainties associated with dose estimations are assessed
to be up to 50% for individual dosimetry, (if the dosimeter was correctly
used), up to a factor 3 to 5 for group and time-and-motion dosimetry
However, the doses do not seem to have been systematically overestimated,
because biological dosimetry performed on limited number of workers
produced results compatible with physical dose estimates.

It is interesting to note that a small special group of 672 scientists
from the Kurchatov Institute who have worked periodically inside the
sarcophagus for a number of years have initially estimated accumulated
whole-body doses in the range 0.5 to 13 Gy (Se95a). These dose estimates
had been reestimated. Recorded and calculated doses available for 501
workers show that more than 20% of them received doses between 0.05
and 0.25 Sv and about 5% of them received doses between 0.25 and 1.5
Sv (Sh97) Additional analysis by mean of FISH technique for three of
them resulted in doses 0.9, 2.0 and 2.7 Sv (Sh00) While no deterministic
effects have been noted to date, this group may well show radiation
health effects in the future.

More than sixteen years after the accident, comparisons between the
different techniques of dosimetry confirm the effectiveness of the chromosome
aberration technique, but indicate that some new methods recommended
by some scientists, such as fluorescent in situ hybridisation (FISH),
do not appear to be a sufficiently sensitive or specific to allow the
estimation of doses for the majority of recovery operation workers (Li98).

The evacuees from the 30-km zone

Immediately after the accident monitoring of the environment was started
by gamma dose rate measurements. About 20 hours after the accident the
wind turned in the direction of Pripyat, gamma dose rates increased
significantly in the town, and it was decided to evacuate the inhabitants.
About 20 hours later the 49 000 inhabitants of Pripyat had left the
town in nearly 1 200 buses. About a further 67 000 people were evacuated
in the following days and weeks (in fact, until September) from the
contaminated areas (a number of 86 000 people given in the NEA's 1996
report (NE95b) was not substantiated).

Information relevant for the assessment of the doses received by these
people have been obtained by 30 000 responses of the evacuees to questionnaires
about the location where they stayed, the types of houses in which they
lived, the consumption of stable iodine, and other activities (Li94).
The average effective dose from external irradiation was estimated to
be 17 mSv, with individual values varying from 0.1 to 380 mSv (Li94).
This value is concordant with the absorbed dose of 20 mGy estimated
by Electron Spin resonance (ESR) measurements of sugar and exposure
rate calculations (Na94).

The main source of uncertainty in the estimation of the average effective
doses from external irradiation is the assessment of the activity ratios
of 132Te and 131I to 137Cs in the deposition.

Doses to the thyroid gland

The iodine activity in thyroid glands of evacuees was measured. More
than about 5 000 measurements of former inhabitants of Pripyat had sufficient
quality to be useful for dose reconstruction (Go95a). A comparative
analysis with the questionnaire responses of 10 000 evacuees showed
that thyroid doses were mainly due to inhalation of 131I. Average individual
doses and collective doses to the thyroid are shown in Table 6 for three
age groups. Individual doses in the age classes were distributed over
two orders of magnitude. The main factor influencing the individual
doses was found to be the distance of the residence from the reactor.

Table 6. Average doses to the thyroid gland and collective thyroid doses
to the evacuees from Pripyat (Go95a)

Year of birth

Number of people

Average individual dose (Gy)

Collective dose (person-Gy)

1983-1986

1971-1982

<1970

2 400

8 100

38 900

1.4

0.3

0.07

3 300

2 400

2 600

Assessments of the doses to the thyroid gland of the evacuees from
the

30-km zone (Li93a) showed similar doses for young children as those
for the Pripyat evacuees. Exposures to adults were higher. These high
doses were due to a greater consumption of food contaminated with 131I
among those evacuated later from the 30-km zone.

Whole-body doses

The whole-body doses to the evacuees were mainly due to external exposure
from deposited 132Te/132I, 134Cs and 137Cs and short lived radionuclides
in the air. Measurements of the gamma dose rate in air were performed
every hour at about thirty sites in Pripyat and daily at about eighty
sites in the 30-km zone. Based on these measurements and using the responses
to the questionnaires, whole-body doses were reconstructed for the 90
000 persons evacuated from the Ukrainian part of the 30-km zone (Li94).
There was a wide range of estimated doses with an average value of 15
mSv. The collective dose was assessed to be 1 300 person-Sv. The 24
000 persons evacuated in Belarus might have received slightly higher
doses, since the prevailing wind was initially towards the north.

The estimates of collective doses for the populations that were evacuated
in 1986 from the contaminated areas of Belarus, Russia and Ukraine was
about 3 800 man Sv for effective dose and 25 000 man Gy for thyroid
doses (UN00). Most of the collective doses were received by the populations
of Belarus and Ukraine.

Because the distributions of iodine tablets was done with a one-week
delay and because only part of the population was covered, the averted
collective thyroid dose from ingestion of contaminated milk was about
30% of the expected collective thyroid dose from that pathway while
the thyroid doses from inhalation remained unchanged.

People living in the contaminated areas

Areas contaminated by the Chernobyl accident have been defined with
reference to the background level of 137Cs deposition caused by the
atmospheric weapons tests, which when corrected for radioactive decay
to 1986, is about 2-4 kBq m-2. considering variations about this level,
it is usual to specify the level of 37 kBq m-2 as the area affected
by the Chernobyl accident.

Approximately 3% of the European part of the former USSR was contaminated
with 137Cs deposition density greater than 37 kBq.m-2. Many people continue
to live in these contaminated territories, although efforts have been
made to limit their doses, 4 400 000 inhabitants were living in areas
with a 137Cs contamination ranging from 37 to 185 kBq.m-2, 580 000 in
areas 185-555 kBq.m-2. Areas of 137Cs deposition density greater than
555 kBq km-2 were designated as areas of strict control. In these areas,
preventive measures have been successfully maintained annual effective
dose below 5 mSv. Because of extensive migration, the number of people
living in these areas under strict control was about 193 000 people
in 1995, down from 273 000 in 1986-1987.

In the first few months, because of the significant release of the
short-term 131I, the thyroid was the most exposed organ, the main route
of exposure was cow-milk pathway. During the first year after the accident,
doses from external irradiation arose from ground deposition of radionuclides
with half-lives of one year or less only in areas close to the reactor,
but the radiocesiums deposition was the greater contributors in more
distant areas only one month after the accident. Over the following
years, the doses received by the populations living in contaminated
areas have come essentially from external exposure due to 134Cs and
137Cs deposited on the soil and internal exposure due to contamination
of foodstuffs by these two isotopes.

A very large number of measurements have been done in the three republics.
The publications prepared for regulatory purposes, tend to over estimate
the average doses that were received during the years 1986-1990.

The main information source for the dose reconstruction is the vast
number of iodine activity measurements of thyroid glands. In Ukraine
150 000 measurements, in Belarus several hundreds of thousands
of measure-ments and in the Russian Federation more than 60 000
measurements were performed in May/June 1986. Some of the measurements
were performed with inadequate instrumentation and measurement conditions
and are not useful for dose assessment purposes. Using these measurements,
the thyroid dose for people who lived in areas where direct thyroid
measurements were done within a few weeks after the accident are being
reconstructed using available data on 131I and 137Cs
deposition.

The influence of having taken stable iodine for prophylactic purposes
has usually not been taken into account in the determination of thyroid
doses (except for the evacuees from Pripyat, iodine prophylaxis was
not effective in reducing the doses substantially as it was done too
late).

The large variability of individual doses makes estimates of dose
distributions difficult and current dose estimates are still subject
to considerable uncertainties, especially in areas where only a few
activity measurements in the thyroid were performed. Children in the
Gomel oblast (region) in Belarus received the highest doses. The distribution
of estimated individual doses in the thyroid of 0-7 years children is
shown in Table 7.

Table 7. Distribution of estimated
individual doses in the thyroid of 0-7 years old children in Gomel
and Mogilev contaminated districts

Gomel

Mogilev

Total

<0.05

784

256

1040

0.05-0.1

527

339

866

0.1-0.3

1762

586

2348

0.3-1

3573

476

4049

1-2

1983

119

2102

>2

5727

44

5771

For the total population of Belarus, the average dose to the thyroid
is 0.9 to 1 Gy for 0-7-year-old children and 0.3 Gy for the total population
giving collective doses of 34 000 and 134 000 man Gy respectively.
(Il91) For the populations of the three republics, the collective thyroid
doses are roughly estimated to 550 000, 200-300 000, 390 000
man Gy for Belarus, Russian Federation and Ukraine respectively (UN00).
The average thyroid dose received by the populations of the three republics
is estimated to be 7 mGy and exceeding 1 Gy for the most exposed children
(UN00). In the eight most contaminated districts of Ukraine where thyroid
measurements were performed, the collective dose to this age group was
about 60 000 person-Sv and for the whole population about 200 000 person-Sv
(Li93). In the Russian Federation the collective dose to the whole population
was about 100 000 person-Sv (Zv93).

The thyroid doses are about two times greater in rural areas than
in urban aeras.

An estimate of the dose distribution among children from Gomel oblast
is shown in Table 8. For the whole Belarus the collective thyroid dose
to children (0 to 14 years) at the time of the accident was assessed
to be about

170 000 person-Gy (Ri94). To day the UNSCEAR report give an estimation
of 34 000 man Gy to 0-7 year old children (UN00).

Table 8. Distribution of thyroid
doses to children (0-15 years)
in the Gomel oblast of Belarus (from UN00)

Limited information is available on in utero thyroid doses.
In a study in 250 children, born between may 1986 and February 1987
in Belarus, thyroid doses were estimated to range up to 4.3 Gy, with
135 children exposed to less than 0.3 Gy, 95 children between 0.3 and
1 Gy, and 20 children with doses greater than 1.Gy (Ig99).

Evaluations of questionnaires on food consumption rates in the period
May/June 1986 and measurements of food contamination showed 131I
in milk as the major source for the thyroid exposure of the population
living in the contaminated areas. However, in individual cases the consumption
of fresh vegetables contributed significantly to the exposure.

Whole-body doses

Two major pathways contributed to the whole-body doses of the population
in contaminated areas, the exposure to external irradiation from deposited
radionuclides (Iv95) and the incorporation into the body of radio-caesium
in food.

The external exposure is directly related to the radionuclide activity
per unit area and it is influenced by the gamma dose rates in air at
the locations of occupancy. Forestry workers and other workers living
in woodframe houses received the highest doses.

Most of the higher contaminated areas are rural and a large part of
the diet is locally produced. Therefore, the uptake of caesium by the
plants from the soil is a deciding factor in the internal exposure.
These are regions with extraordinarily high transfer factors, as the
Rovno region in Ukraine, where even moderate soil contamination led
to high doses. In order of decreasing magnitude of transfer factors
these regions are followed by regions with peaty soil, sandy podzol
(acidic infertile forest soil), loamy podzol, and chernozem which is
rich black soil.

In the first years after the accident the caesium uptake was dominated
practically everywhere by the consumption of locally produced milk (Ho94).
However, later mushrooms began to contribute significantly in many settlements
to the caesium incorporation for two reasons. First, the milk contamination
decreased with time, whereas the mushroom contamination remained relatively
constant. Second, due to changes in the economic conditions in the three
republics, people are collecting more mushrooms than they were in the
first years after the accident.

The normalised lifetime doses for urban and rural populations of the
three republics is now estimated to range from 42 to 88 μSv per
kBqm-2 of 137Cs, 60% being received during the
first 10 years. These values are lower than the first estimates, because
they are more realistic and take account of, for example, the vertical
migration of cesium in soils. During the first 10 years after the accident,
average effective doses in theses areas ranged from 5mSv in Russian
urban areas to 11 mSv in the rural areas of Ukraine. The variability
of dose distribution could be represented by a log-normal distribution
with a geometric standard deviation of 1.54. The decontamination measures
had a limited impact on members of population. It was expected that
less than 15% of the dose could be averted for the general population,
and only 35% for school children. The total averted collective dose
attributable to decontamination procedures was estimated to 1 500
mSv for the first four years.

The distribution of the collective dose from external irradiation
by region and dose interval are presented in Table 9 and 10.

Table 10. Distribution of estimated
total effective doses received by the populations of contaminated areas
(1986-1995) excluding thyroid dose

Number of persons

Dose interval (mGy)

Belarus

Russian Federation

Ukraine

<1

133 053

155 301

-

1-5

1163 490

1 253 130

330 900

5-20

439 620

474 176

807 900

20-50

113 789

82 876

148 700

50-100

25 065

14 580

7 700

100-200

5 105

2 979

400

>200

790

333

-

Table 11 summarises an estimate of whole-body doses to people living
in the higher contaminated areas. On average, external irradiation was
by far the highest contributor to the total population exposure (Er94).
However, the highest doses to individuals were produced by the consumption
of food from areas with high transfers of radionuclides.

Table 11. Distribution of external
and total whole-body doses during 1986-89, to inhabitants of contaminated
areas (137Cs activity per unit
area >555 kBq/m2) (Ba94)

Whole-body dose (mSv)

External exposure

Total exposure

No. of persons

Collective dose (man.Sv)

No. of persons

Collective dose (man.Sv)

5-20

20-50

50-100

100-150

150-200

>200

132 000

111 000

24 000

2 800

530

120

1 700

3 500

1 600

330

88

26

88 000

132 000

44 000

6 900

1 500

670

1 200

4 200

3 000

820

250

160

Total

270000

7300

273000

9700

The total collective effective dose received during the first 10 years
after the accident by the approximately 5.2 million people living in
the contaminated areas of Belarus, the Russian federation and Ukraine
is estimated to be 24 200 man.Sv. This means, as ten years
represents 60%, that the lifetime collective dose from external irradiation
would be 40 300 man.Sv (UN00).

Internal doses were 5 500 man Sv for Belarus, 5 000 man.Sv
for the Russian Federation an 7 900 man.Sv for Ukraine. As 10 years
represents 90%, the lifetime total for the three republics would be
20 400 man.Sv, corresponding to an average individual lifetime
effective dose of 3.9 mSv [UN00].

Total collective thyroid doses in Belarus, the Russian Federation,
and the Ukrain, respectivly, were estimated to be 550 000 250 000
and 740 000 man Gy.

The total of about 60 700 man Sv for external and internal doses
corresponds to an average individual lifetime effective dose of 12 mSv,
excluding thyroid collective dose delivered during the first year. This
is estimated to be 1 500 000 man Gy in total for the three
countries.

Populations outside the former Soviet Union

Even though the releases of radioactive materials during the Chernobyl
accident mainly affected the populations of Belarus, Russia and Ukraine,
the released materials became further dispersed throughout the atmosphere
and the volatile radionuclides of primary importance (131I
and 137Cs) were detected in most countries of the Northern
hemisphere. However, population doses were, in most places, much lower
than in the contaminated areas of the former Soviet Union; they reflected
the deposition levels of 137Cs and were higher in areas where
the passage of the radioactive cloud coincided with rainfall. Generally
speaking, however, and with a few notable exceptions, the doses decreased
as a function of distance from the reactor (Ne87).

During the first few weeks after the accident, 131I was
the main contributor to the dose, via ingestion of milk (Ma91). Infant
thyroid doses generally ranged from 1 to 20 mGy in Europe, from
0.1 to 5 mGy in Asia, and were about 0.1 mGy in North America. Adult
thyroid doses were lower by a factor of about 5 (UN88).

Later on, 134Cs and 137Cs were responsible for
most of the dose, through external and internal irradiation (Ma89).
The whole-body doses received during the first year following the accident
generally ranged from 0.05 to 0.5 mGy in Europe, from 0.005 to 0.1 mGy
in Asia, and of the order of 0.001 mGy in North America. The total
whole-body doses expected to be accumulated during the lifetimes of
the individuals are estimated to be a factor of 3 greater than the doses
received during the first year (UN88).

In summary

A large number of people received substantial doses as a result of
the Chernobyl accident:

Liquidators - Hundreds of thousands of workers,
estimated to amount up to 600 000, were involved in clean-up
operations. The most exposed, with doses of several grays, were the
workers involved immediately after the beginning of the accident and
the scientists who have performed special tasks in the sarcophagus.
The average doses to liquidators are reported to have ranged between
170 mSv in 1986 and 15 mSv in 1989.

Evacuees - More than 100 000 persons were
evacuated during the first few weeks following the accident. The evacuees
were exposed to internal irradiation arising from inhalation of radioiodines,
especially 131I, and to external irradiation
from radioactivity present in the cloud and deposited on the ground.
Thyroid doses are estimated to have been, on average, about 1 Gy for
small children under 3 years of age at the time of the accident, and
about 70 mGy for adults. Whole-body doses received from external irradiation
prior to evacuation from the Ukrainian part of the 30-km zone showed
a large range of variation with an average value of 15 mGy.

People living in contaminated areas of the former
Soviet Union - About 270 000 people live in contaminated
areas with 137Cs deposition levels in excess of 555 kBq/m2.
Thyroid doses, due mainly to the consumption of cow's milk contaminated
with 131I, were delivered during the first few weeks after
the accident; children in the Gomel region of Belarus appear to have
received the highest thyroid doses with a range from negligible levels
up to 40 Gy and an average close to 1 Gy for children aged 0 to 7.
Thanks to the control of foodstuffs in those areas, most of the radiation
exposure has been due to external irradiation from the 137Cs
activity deposited on the ground; the whole-body doses for the 1986-1989
time period are estimated to range from 5 to 250 mGy with an average
of 40 mGy. In areas without food control, there are places, such as
the Rovno region of Ukraine, where the transfer of 137Cs
from soil to plant is very high, resulting in doses from internal
exposure being greater than those from external exposure.

Populations outside the former Soviet Union
- The radioactive materials of a volatile nature (such as iodine and
caesium) that were released during the accident spread throughout
the entire northern hemisphere. The doses received by populations
outside the former Soviet Union were relatively low, and showed large
differences from one country to another depending mainly upon whether
rainfall occurred during the passage of the radioactive cloud.

*Special thanks to Dr. André Bouville, of the US National Cancer
Institute, for his verification of the facts in this chapter.